A coating composition for top coating an automobile generally contains, as a film forming binder, a combination of a hydroxyl group-containing polymer and a melamine curing agent. The cured film obtained from the melamine curing system, however, has poor acid resistance and is damaged by acid rain which has recently become one of serious problems. The damage of the film provides poor appearance.
In order to overcome the above mentioned defects, the present inventors have proposed novel coating compositions without employing the melamine curing agent in Japanese Kokai Publications 45577/1990, 287650/1991. The similar coating compositions are also disclosed in U.S. Pat. No. 2,977,334 (to Zopf et al.), U.S. Pat. No. 3,399,109 (to Zimmerman et al.) and U.S. Pat. No. 3,528,935 (to Marion et al.). The proposed coating compositions cure by means of a reaction of acid with epoxy and therefore has good acid resistance and sufficient weather resistance for an automotive top coating application.
However, these coatings have a problem that they are easily brushed by a car washing machine, because of their poor mar resistance. If degree of crosslinking is raised in order to impart good mar resistance at the mechanical car washing in this curing system, the resulting cured film has poor extensibility and is stiff and brittle. The stiff and brittle film brings another problems that it easily cracks, particularly when it is formed on a sealer, because such a film hardly responds to expansion and contraction of the sealer. It is therefore difficult to use the proposed coating compositions as a top coating composition for the automobiles. It is desired to develop top coating composition which has not only excellent acid resistance but also excellent mar resistance and what we call "sealer cracking resistance".